I am not claiming to be an expert, but I have done some diet formulation for my animal nutrition and metabolism classes.
The purina cat chow will not be as good as the purina fish chow.
Sure, the digestibility and % protein could be the same, but if the protein composition does not contain the proper "building blocks", then it will reflect poorly on growth.
Have you entertained the idea of growing some aquatic plants for the tilapia to eat? It might be a good supplement to their diet.
